Big hatch went south towards end, why?

I set 108 eggs. Culled about 12 at day 7 for clears or blood rings. Culled again at day 11, 13 this time. One last cull at 14, down to 67 now, not great but still at +50%. I now have about 18 that are hatching rest dead. My question is what would cause so many late deaths? Almost all are well formed. The eggs are clean, I didnt wash them. Not my first hatch with this incubator, I have done well with it before. I suspect I may not have adequate ventalation, there is a 1.25" square hole on back and a fan on the inside.

Thoughts?

No odor from the dead, at all.

 

It was my own eggs, which I have hatched with before under broodies and in this bator. It is a cooler bator that I built out of a 150 quart cooler. I run it around 30% humidity the first 18 days then add water at 18. I have had the fan give me some issues, temp got to 102 a few times but did not stay there, usually run at 100. I had eggs in trays mad frim 18ct cartons with bottoms cut off, pointy end down on eggs. Turned at least twice daily by tilting trays opposite ends up each time. all babies that I checked frim bad eggs seemed to be positioned correctly. I did have two days oit of town that I wasnt able to turn. Id like to try to narrow this down as it is very discouraging to see this many babies die. I want to try again but with these results I am hesitant too with such a bad turn out. I want to point my finger at ventilation since so many died towards the end, but i would like opinion s of the more experienced folks. Www